1. Field of the Invention
The present invention relates to a technique of annealing, for instance, a semiconductor material by illuminating it with laser light. The invention generally relates to techniques of processing or modifying an object in various manners by illuminating it with laser light.
The invention also relates to a laser annealing apparatus and method for annealing a semiconductor material by using a linear laser beam.
The invention is particularly effective when used, for instance, in a process of converting an amorphous silicon film into a crystalline silicon film, a process of improving the crystallinity of a crystalline silicon film, and a process of repairing lattice defects that have been generated by implanting an impurity into a crystalline silicon film to, for instance, render it conductive all of which processes are performed by laser annealing.
2. Description of the Related Art
In recent years, various studies have been made extensively to reduce the temperature of semiconductor device manufacturing processes. The major reason for this tendency is the need of forming semiconductor devices on an insulative substrate, such as a glass substrate, which is inexpensive and highly workable. Stated more specifically, this is due to the need of forming thin-film transistors of several hundred by several hundred or more on a glass substrate in producing an active matrix liquid crystal display device. Other needs such as the needs of forming finer devices and multilayered devices have also prompted the studies mentioned above.
In semiconductor manufacturing processes, it is sometimes necessary to crystallize an amorphous semiconductor material or amorphous components contained in a semiconductor material, recover the crystallinity of a semiconductor material which was originally crystalline but has been lowered in the degree of crystallinity due to ion irradiation for impurity implantation, or improve the degree of crystallinity of an already crystalline semiconductor material. Conventionally, thermal annealing is used for these purposes. Where the semiconductor material is silicon, crystallization of amorphous silicon, recovering or improvement of crystallinity, etc. are attained by performing annealing at 600 to 1,100xc2x0 C. for 0.1 to 48 hours or more.
In general, the above-mentioned thermal annealing may be performed in a shorter processing time when the temperature is higher. However, it has almost no effect when the temperature is 500xc2x0 C. or less. Therefore, from the viewpoint of decreasing the temperature of a process, it is necessary to replace a step that conventionally uses thermal annealing with some other means.
In particular, where a glass substrate is used, it is required that the thermal annealing temperature be 700xc2x0 C. or less, and that the heating time be as short as possible. The latter requirement is due to the fact that a long heat treatment may deform the glass substrate. In a liquid crystal display device, a liquid crystal is held between a pair of glass substrates having a gap of several micrometers. Therefore, deformation of the glass substrates greatly affects display performance of the liquid crystal display device.
Various types of annealing technique using laser light illumination are known as processes for replacing the thermal annealing. Laser light can impart high energy that is equivalent to the energy obtained by the thermal annealing only to a desired portion; it is therefore not necessary to expose the entire substrate to a high-temperature atmosphere.
Stated in general, there have been proposed the following two laser light illumination methods:
In the first method, a CW laser such as an argon ion laser is used and a spot-like beam is applied to a semiconductor material. A semiconductor material is crystallized such that it is melted and then solidified gradually due to a sloped energy profile of a beam and its movement.
In the second method, a pulsed oscillation laser such as an excimer laser is used. A semiconductor material is crystallized such that it is melted instantaneously by application of a high-energy laser pulse and then solidified.
The first method has a problem of long processing time, because the maximum energy of a CW laser is insufficient and therefore the beam spot size is at most several square millimeters. In contrast, the second method can provide high mass-productivity, because the maximum energy of a laser is very high and therefore the beam spot size can be made several square centimeters or larger.
However, in the second method, to process a single, large-area substrate with an ordinary square or rectangular beam, the beam needs to be moved vertically and horizontally, which inconvenience still remains to be solved from the viewpoint of mass-productivity.
This aspect can be greatly improved by deforming a laser beam into a linear shape and moving the linear beam approximately perpendicularly to its longitudinal direction to effect scanning. The term xe2x80x9cscanningxe2x80x9d as used in this specification means illuminating an object while moving a linear laser beam step by step with an overlap in the beam width direction, that is, approximately perpendicularly to the longitudinal direction of the beam.
The problem remaining unsolved is insufficient uniformity of laser light illumination effects. The following measures have been taken to improve the uniformity. A first measure is to make the beam profile as close to a rectangular one as possible by causing a laser beam to pass through a slit, to thereby reduce an energy variation within a linear beam.
FIGS. 4A and 4B show an energy profile of a laser beam; FIG. 4A shows an example of a rectangular energy profile. The term xe2x80x9crectangularxe2x80x9d as used in this specification means a relationship L2, L3xe2x89xa60.2L1 where L1 to L3 are defined in FIG. 4B.
In using the above technique, it has been reported that the uniformity can further be improved by performing preliminary illumination with weaker pulse laser light before illumination (hereinafter called xe2x80x9cmain illuminationxe2x80x9d) with stronger pulse laser light.
This measure is so effective that the characteristics of resulting semiconductor devices can be improved very much. This is because the two-step laser light illumination with different illumination energy levels allows a semiconductor film to be crystallized step by step, thereby reducing the seriousness of such problems as a non-uniform distribution of crystallinity, formation of crystal grains, and concentration of stress, which problems result from abrupt phase changes.
The stepped illumination can be made more effective by increasing the number of illumination steps.
Thus, the above two kinds of measure can greatly improve the uniformity of the laser light illumination effects.
However, with the above two-step illumination method, the laser processing time is doubled, that is, the throughput is reduced.
Further, the equipment for the two-step illumination method is more complex than that for the single step illumination method, thus causing a cost increase.
In addition, although the above measures have much improved the uniformity of the laser light illumination effects, the degree of improvement is still insufficient.
To transform a square or rectangular light beam into a linear beam, a specialized optical system is needed.
FIG. 14 shows an example of an optical system of a conventional laser annealing apparatus.
The optical system of FIG. 14 is composed of the following components. An excimer laser beam generating mean Axe2x80x2 generates an excimer laser beam. Beam expanders Bxe2x80x2 and Cxe2x80x2 expand the excimer laser beam. A vertical expansion fly-eye lens Dxe2x80x2 and a horizontal expansion fly-eye lens D2xe2x80x2 expand the laser beam in a sectional manner. A first cylindrical lens Exe2x80x2 converges the laser beam into a line shape. A second cylindrical lens Fxe2x80x2 improves the uniformity of the linear laser beam in its longitudinal direction. A stage Ixe2x80x2 is moved in direction Jxe2x80x2 indicated by an arrow in FIG. 14 in a state that an illumination object, a substrate bearing an illumination object, or the like is placed thereon.
In FIG. 14, a path-folding mirror Gxe2x80x2 and a cylindrical lens Hxe2x80x2 serve to apply the laser beam to an object on the stage Ixe2x80x2. In certain types of configuration, the beam expanders Bxe2x80x2 and Cxe2x80x2 are omitted.
A uniform linear laser beam can be obtained by the above optical system. However, in this conventional optical system, the use of two fly-eye lenses for sectionally expanding a laser beam, that is, the fly-eye lens Dxe2x80x2 for vertical expansion and the fly-eye lens D2xe2x80x2 for horizontal expansion, lowers the transmittance of the entire fly-eye lens system, resulting in a low laser beam energy efficiency. As a result, in laser annealing, the amount of energy applied to an illumination object may be lowered, possibly making the annealing insufficient.
To prevent this problem, the output of the laser light source needs to be increased. But this increases the load on the laser light source, so that the life of the entire apparatus may be shortened.
In view of the above, a first object of the present invention is to obtain highly uniform laser light illumination effects in crystallizing a semiconductor coating by using a linear laser beam emitted from a pulsed laser.
In particular, it is an object of the invention to obtain highly uniform laser light illumination effects by single step illumination, that is, without using a two-step scheme consisting of preliminary illumination and main illumination.
A second object of the invention is to provide a laser annealing apparatus and method which are intended to generate a uniform linear laser beam for use in laser annealing particularly to crystallize an amorphous silicon film formed on an insulative substrate such as a glass substrate, or improve the crystallinity of a thermally crystallized silicon film formed on an insulative substrate such as a glass substrate, and in which apparatus and method an optical system used is low in energy loss and capable of applying sufficient energy to an illumination object, and a laser light source has a long life.
The invention attains the first object by properly adjusting the energy profile of a linear laser beam. More specifically, the invention causes a linear laser beam to have, at its focus, a quasi-trapezoidal energy (density) profile in its width direction (i.e., laser beam scanning direction).
Processing such as crystallization is performed by applying a laser beam having the above energy profile to a semiconductor material coating while scanning the coating with the laser beam.
Major aspects of the invention will be described below.
According to one of the major aspects of the invention, there is provided a laser annealing method in which a linear laser beam emitted from a pulsed laser light source is applied to an illumination surface that is a semiconductor coating, wherein:
the linear laser beam has, at a focus, an energy profile in a width direction thereof which satisfies inequalities 0.5L1xe2x89xa6L2xe2x89xa6L1 and 0.5L1xe2x89xa6L3xe2x89xa6L1 where assuming that a maximum energy is 1, L1 is a beam width of two points having an energy of 0.95 and L1+L2+L3 is a beam width of two points having an energy of 0.70, L2 and L3 occupying two peripheral portions of the beam width.
According to another aspect of the invention, there is provided a laser annealing method in which a linear laser beam emitted from a pulsed laser light source is applied plural times to an illumination surface that is a semiconductor coating while the linear laser beam and the illumination surface are moved relative to each other in a width direction of the linear laser beam, wherein:
the linear laser beam has, at a focus, an energy profile in a width direction thereof which satisfies inequalities 0.5L1xe2x89xa6L2xe2x89xa6L1 and 0.5L1xe2x89xa6L3xe2x89xa6L1 where assuming that a maximum energy is 1, L1 is a beam width of two points having an energy of 0.95 and L1+L2+L3 is a beam width of two points having an energy of 0.70, L2 and L3 occupying two peripheral portions of the beam width.
FIG. 5 illustrates how a linear laser beam having a quasi-trapezoidal energy profile is applied.
Referring to FIG. 5, pulse laser beams having an energy density profile as shown in FIG. 4B (the beam width is defined as a half width of a maximum energy value of a laser beam) are applied while being moved gradually with overlaps. In this case, a linear region at a particular location is illuminated with plural pulses. During this illumination with plural pulses, the illumination energy density of pulses increases in a step-like manner at the first stage and then decreases also in a step-like manner.
That is, the invention is characterized in that in applying linear pulse laser beams while moving those in one direction, they are applied in an overlapped manner so that an arbitrary point on an illumination object is illuminated with pulse laser beams plural times, that is, 3 to 100 times, preferably 10 to 40 times.
In the above aspects of the invention, which are intended to attain the first object, in applying linear pulse laser beams while moving those in their width direction, they are given a quasi-trapezoidal energy profile in the width direction.
The quasi-trapezoidal energy profile means a profile that satisfies inequalities 0.5L1xe2x89xa6L2xe2x89xa6L1 and 0.5L1xe2x89xa6L3xe2x89xa6L1 where assuming that a maximum energy is 1, L1 is a beam width of two points having an energy of 0.95 and L1+L2+L3 is a beam width of two points having an energy of 0.70, L2 and L3 occupying two peripheral portions of the beam width.
When linear laser beams having the above energy profile are applied while moving those, an arbitrary point in an illumination region is first illuminated with low-energy-density laser beams corresponding to the bottom portion of the trapezoidal energy profile. As laser beams are moved subsequently, the energy density gradually increases, and laser beams having an energy density corresponding to the top base (having a maximum value) of the trapezoidal energy profile come to be applied. Finally, the energy density gradually decreases.
In this manner, an arbitrary point in the illumination region is illuminated with laser beams whose energy density varies continuously so as to correspond to the trapezoidal energy profile.
Therefore, the bottom portions having an energy gradient of the above trapezoidal energy profile substantially has the role of the preliminary illumination of weak laser light energy of the above-mentioned two-step laser light illumination that consists of the preliminary illumination and the main illumination. Thus, the invention can provide the same effects as in the case of changing the illumination energy in a step-like manner.
That is, a situation equivalent to the situation in which an arbitrary point in an illumination region is first illuminated with weak laser beams, then laser beams whose intensity is gradually increased and then laser beams whose intensity is gradually decreased, and the illumination is finished can be realized by applying laser beams in the above-described manner rather than using the two-step illumination.
With the above laser light illumination, since the energy supplied to an illumination region does not vary abruptly, abrupt phase changes etc. can be prevented from occurring in the illumination object.
Therefore, for instance in crystallizing an amorphous semiconductor by illuminating it with laser light, by virtue of the absence of abrupt phase changes, there does not occur surface roughening or accumulation of internal stress, enabling a uniform distribution of crystallinity, that is, uniform annealing effects.
Further, the illumination with the trapezoidal energy profile makes the depth of focus of a laser beam wider than that of a conventional laser beam, thereby facilitating laser processing.
In contrast to the fact that a conventional laser beam having the rectangular energy profile has a depth of focus of about xc2x1200 xcexcm, a laser beam having the trapezoidal energy profile that satisfies 0.5L1xe2x89xa6L2xe2x89xa6L1 and 0.5L1xe2x89xa6L3xe2x89xa6L1 provides a depth of focus of about xc2x14.00 xcexcm.
FIG. 7 schematically shows a relationship between the laser beam energy profile and the depth of focus (absolute value). A hatched region b corresponds to the laser beam energy profile of the invention which satisfies 0.5L1xe2x89xa6L2xe2x89xa6L1 and 0.5L1xe2x89xa6L3xe2x89xa6L1. The horizontal axis represents L2/L1 (or L3/L1). As this value approaches 0, the laser beam energy profile becomes closer to a rectangle. Conversely, as this value becomes larger, the energy profile comes to assume a trapezoid or triangle.
A wide depth of focus of a laser beam allows laser processing to be performed uniformly even on an illumination surface having a certain degree of undulation or asperities.
For example, after a 0.2-xcexcm-thick silicon oxide film and a 0.1-xcexcm-thick amorphous silicon film are sequentially deposited on a glass substrate and thermal crystallization is performed at 600xc2x0 C., the glass substrate is likely to have undulation of plus and minus several tens of micrometers to several hundred micrometers if it is about 300xc3x97300 mm2 in size.
In such a case, a laser beam having the conventional rectangular profile of region a of FIG. 7 (0.5L1 greater than L2, L3) has a depth of focus of about xc2x1200 xcexcm, non-uniform crystallization occurs in the amorphous silicon film. As a result, a crystallized silicon film likely has a mobility variation as large as more than 10% in the substrate area.
In contrast, in region c (L2, L3 greater than L1), where the depth of focus is too wide, the focus adjustment becomes difficult and the energy density imparted to an illumination object becomes too low. As a result, the crystallization of the amorphous silicon film becomes insufficient and a desired mobility is not obtained.
A laser beam produced in the above manner has a depth of focus of about xc2x1400 xcexcm, and therefore is more resistant, by a factor of about 2 to 8, to undulation of a substrate or coating than a conventional laser beam. Thus, laser processing on a silicon film having asperities of the above kind can be performed very uniformly at a sufficiently high energy density.
Therefore, even a silicon film formed on a substrate having undulation of several hundred micrometers can be processed to have a uniform mobility distribution having a variation of less than 10% and sufficiently large mobility values.
As such, a laser beam having the trapezoidal energy profile of the invention can provide very uniform laser light illumination to even an illumination surface, such as a semiconductor coating, having undulation or asperities.
The above effects become more effective as the substrate size becomes larger.
Since the above aspects of the invention provides a depth of focus of about xc2x1400 xcexcm, it enables uniform crystallization on an illumination object whose asperities are less than about xc2x1400 xcexcm.
With the depth of focus of the above level, in crystallizing a silicon coating by laser light illumination, the crystallization can be made so uniform that a mobility variation of the coating falls within xc2x110%.
It is noted that the above values are ones obtained in a case where a shot-by-shot energy variation of pulse laser beams falls within xc2x13% in terms of 3"sgr". Where pulse laser beams have an energy variation that is equal to or larger than xc2x13% in terms of 3"sgr", the depth of focus is reduced. Pulse laser beams having an energy variation that is equal to or larger than xc2x110% in terms of 3"sgr" are not suitable for crystallization of a semiconductor.
To attain the second object, according to another aspect of the invention, there is provided a laser annealing apparatus comprising (see FIG. 9):
pulse laser beam generating means (K) for generating a pulse laser beam;
beam expanders (L, M) for expanding the generated laser beam;
a compound-eye-like fly-eye lens (N) for expanding, sectionally, the expanded laser beam;
a first cylindrical lens (O) for converging the sectionally expanded laser beam into a linear laser beam;
a second cylindrical lens (P) for improving uniformity of the linear laser beam in a longitudinal direction thereof; and
a stage (S) for moving an illumination object relative to the linear laser beam approximately perpendicularly to the longitudinal direction thereof.
According to a still another aspect of the invention, there is provided a laser annealing apparatus comprising (see FIG. 10):
pulse laser beam generating means (k) for generating a pulse laser beam;
a compound-eye-like fly-eye lens (l) for expanding, sectionally, the pulse laser beam;
a first cylindrical lens (m) for converging the sectionally expanded laser beam into a linear laser beam;
a second cylindrical lens (n) for improving uniformity of the linear laser beam in a longitudinal direction thereof; and
a stage (q) for moving an illumination object relative to the linear laser beam approximately perpendicularly to the longitudinal direction thereof.
In the above configurations, it is preferred that the pulse laser beam generating means be excimer laser beam generating means.
It is preferred that a slit for eliminating a peripheral portion of the linear laser beam be provided downstream of the first cylindrical lens.
It is also preferred that the compound-eye-like fly-eye lens be configured such that a plurality of convex lenses each having a polygonal sectional shape are arranged regularly and adjacently into a planar shape. It is preferred that each convex lens has a square, rectangular, hexagonal, or like sectional shape.
To attain the second object, according to a further aspect of the invention, there is provided a laser annealing method comprising the steps of:
expanding, sectionally, a pulse laser beam with a compound-eye-like fly-eye lens;
converging the sectionally expanded laser beam into a linear laser beam; and
illuminating and scanning an illumination object with the linear laser beam.
In the above method, it is preferred that the pulse laser beam be an excimer laser beam.
In the above laser annealing apparatus and method, a laser beam as generated by the excimer laser beam generating means or a laser beam thus generated and then expanded and shaped by the beam expanders is expanded, in a sectional manner, by the single compound-eye-like fly-eye lens.
With this configuration, the loss of light quantity is reduced, compared with the case of using two fly-eye lenses for vertical expansion and horizontal expansion. As a result, the loss of the laser beam energy is greatly reduced, that is, the energy efficiency is improved. This makes it possible to provide superior laser annealing and crystallization of a silicon film, and to elongate the life of a laser light source.
FIG. 8 shows an example of the compound-eye-like fly-eye lens. The compound-eye-like fly-eye lens of the invention is constructed by arranging, regularly and adjacently, a plurality of convex lenses 801 each having a polygonal, for instance, square, sectional shape into a planar shape. This compound-eye-like fly-eye lens has a function of uniformly expanding, in a sectional manner, incident light in both vertically and horizontally, though it is a single lens.
It is preferable that the individual convex lenses constituting the fly-eye lens assume a polygon, in particular, a rectangle, square, hexagon, or the like. This is because in such a case they can easily be arranged regularly, and hence the fly-eye lens can be formed and worked easily. Further, the fly-eye lens can easily be given high precision.
The lenses that were mentioned above as the components of the laser annealing apparatus serve to converge a laser beam into a linear beam and to make the beam energy profile uniform in the width direction. After a laser beam is expanded by the beam expanders and/or the fly-eye lens, it is converged into a linear beam by a rod-shaped converging lens that is cylindrical in one direction, for instance, a cylindrical lens.
Immediately after emission, an excimer laser beam as a pulse laser beam has a rectangular cross-section and a generally uniform intensity distribution in the cross-section.
The beam expanders increases the width of the laser beam, and expands and shapes the beam cross-section into a square (or rectangular) shape, thus increasing the cross-sectional area.
However, the use of the beam expanders reduces the energy efficiency as much as the increase in the number of lenses. Therefore, the beam expanders may be omitted.
In addition to expanding the beam area, the fly-eye lens has a function of making the beam energy profile uniform. It is noted that originally the fly-eye lens was developed to provide a uniform beam.
Due to the spherical aberration, a laser beam as converged into a linear shape has an energy profile in the width direction which includes low energy density portions at beam peripheries like those of a Gaussian distribution. Therefore, the peripheral portions of a linear laser beam do not end in a definite manner.
In view of the above, a proper slit may be used to cut the peripheral portions (bottom portions) of the Gaussian-distribution-like energy profile in the width direction which portions occur in the linear laser beam after passage through the cylindrical lens.
Laser beams are applied, with proper overlaps, to an illumination object such as an amorphous silicon film formed on a glass substrate which object is placed on a stage while the stage is moved. In this manner, the amorphous silicon film and the like can be crystallized uniformly at high speed.
The laser annealing apparatus and method of the invention are particularly effective when they are applied to, for example, a step of converting an amorphous silicon film into a crystalline silicon film by laser annealing, a step of improving the crystallinity of a crystalline silicon film, and a step of repairing lattice defects that occur when impurity ions have been implanted into a crystalline silicon film to, for instance, render it conductive.
In particular, the above apparatus and method are effectively applied to various kinds of film formed on a glass substrate.